A critical vulnerability found in a WordPress plug-in that has been downloaded over 1.7 million times allows potential attackers to take complete control of blogs that use it.

The flaw is located in the MailPoet Newsletters plug-in, previously known as wysija-newsletters, and was discovered by researchers from Web security firm Sucuri.

“This bug should be taken seriously; it gives a potential intruder the power to do anything he wants on his victim’s website,” Daniel Cid, Sucuri’s chief technology officer, said in a blog post Tuesday. “It allows for any PHP file to be uploaded. This can allow an attacker to use your website for phishing lures, sending SPAM, hosting malware, infecting other customers (on a shared server), and so on!”

The vulnerability was patched in MailPoet version 2.6.7, released Tuesday, so all WordPress blog administrators should upgrade the plug-in to the latest version as soon as possible if they use it.

The flaw was the result of the MailPoet developers wrongly assuming that the “admin_init” hook in WordPress is only triggered when an administrator visits pages from the administration panel, Cid said.

The MailPoet developers used admin_init to verify whether the active user is allowed to upload files, but since this hook is actually also triggered by a page accessible to unauthenticated users, the plug-in’s file upload functionality was made available to virtually anyone.

It’s easy to make this mistake and all plug-in developers should be mindful of this behavior, Cid said. “If you are a developer, never use admin_init() or is_admin() as an authentication method.”

WordPress sites are a constant target for attackers and those that get compromised are frequently used to host spam pages or malicious content as part of other attacks. Cybercriminals are running scans on the Internet every day to identify WordPress installations affected by vulnerabilities like the one found in MailPoet.